The present invention relates to a method and apparatus for investigating the deposition and retention of particulate material on surfaces.
There have been various proposals for determining the amount of airborne particulate materials present in the air in various environments. Thus U.S. Pat. No. 3,572,128 discloses a polished metal disk exposed to the air on which dust can settle. Periodically all the dust on the surface is wiped off and concentrated by suction in order to determine the quantity of dust deposited.
It is known to treat hard surfaces with products such as furniture polish to clean them and impart desirable properties such as increased shine to the surface. Such surfaces are exposed to airborne dust. Some dust particles may be electrically charged and some products for cleaning hard surfaces claim to reduce the static charge on the surface and so to reduce the tendency of the surface to attract dust. However dust will still be deposited on substantially horizontal surfaces by the action of gravity. The nature of any treatment applied to the surface may affect the strength with which the dust is bound to the surface and its ease of removal in any subsequent cleaning operation. In such a situation the information of primary interest is not the amount of dust in the atmosphere which is deposited over a period of time on the surface but the properties of the dust on the surface.
The applicant has understood that there is a need to be able to compare the effects of different treatments on the ability of a hard surface to retain dust and has provided a method and apparatus for carrying out such comparisons.
According to the present invention there is provided a process for comparing dust retention properties of a first surface of a substrate with the dust retention properties of a second surface of a substrate,which comprises, within a dust-retaining enclosure,
a) subjecting said first surface, previously coated with a standardized layer of dust, to a standardized dust dislodging force sufficient to dislodge dust from at least one of the surfaces.
b) determining any increase in the quantity of dust particles in the atmosphere within the enclosure as a result of the dust dislodging force,
c) removing any excess dust particles resulting from the application of the dust dislodging force from the atmosphere in the enclosure,
d) subjecting said second surface, previously coated with a standardized layer of dust, to a standardized dust dislodging force,
e) determining any increase in the quantity of dust particles in the atmosphere within the enclosure as a result of the dust dislodging force, and
f) comparing the relative amounts of dust particles found by steps b) and e).
The applicant has realized that instead of examining the surfaces directly, it is possible to study the atmosphere above the surface, making possible a simple test method using readily available components.
According to a further aspect of the invention there is provided an apparatus suitable for use in comparing the dust retention properties of surfaces, said apparatus comprising
a) a dust-retaining enclosure,
b) substrate introduction means for introducing a substrate into the enclosure, said substrate having a surface exposed to the atmosphere within the container when within the container,
c) manipulation means for allowing manipulation of the substrate within the enclosure from outside the enclosure,
d) gas stream means for introducing a stream of gas into the enclosure adjacent to said surface of the substrate when the substrate is within the enclosure,
e) dust measurement means for measuring the dust content in the atmosphere within the enclosure.
Preferably, the apparatus also has dust distribution means, said dust distribution means comprising a dust receiving means within the enclosure and air distribution means for passing air over the dust receiving means.
The substrate introduction means may be a portion of the enclosure which may be moved to allow access to the interior of the enclosure.
The manipulation means may be an opening or openings through which an operator""s hand can be inserted and which can be blanked off when not required.
The first and second surfaces may be provided on different substrates which are introduced separately into the enclosure with the dust dislodging step being applied separately to each substrate in the absence of the other. Where two substrates are used it is preferred to have both present in the enclosure simultaneously, and for suitable shielding to be provided to protect one substrate while the dust dislodging step is applied to the other.
The present invention may be used to compare the inherent dust retention properties of two different materials, in which case the two surfaces will be the surfaces of two different substrates. The invention may also be used to compare the effects of different surface treatments applied to the same material. In such a case two different substrates of the same material may be used. It may be more convenient to provide the surfaces to be compared on a single substrate.
The optimum area of each surface to be investigated will depend on the size of the enclosure but may for example be in the range 200-1000 cm2, e.g. 600 cm2.
The enclosure used in the method must retain at least a substantial proportion of the dust released from the surfaces under investigation, so that sufficient remains in the atmosphere within the enclosure to be measured. The size of the enclosure is desirably selected in relation to the size of the surface which carries dust so as to give a measurable increase in dust levels within the atmosphere within the enclosure. An enclosure the size of a mobile fume cupboard with a volume of less than 1 cubic metre is suitable.
The surfaces to be compared require to be coated with a standardized layer of dust so that the results for dust displaced into the atmosphere of the enclosure are comparable. This standardized layer may be deposited outside the enclosure. Thus the surfaces to be compared may be left exposed to airborne dust in the same room for the same period of time. It is preferred, however, to deposit the dust layer on the surface after the substrate has been introduced into the enclosure. In order to deposit the dust layer on the substrate within the enclosure it may be desirable to provide the enclosure with a dust receiving means for receiving a sample of dust and air distributing means for passing air over the dust receiving means so as distribute the dust into the air within the enclosure from which it can settle on to the substrate. The air distribution means may be a fan which draws air into the enclosure from outside. The dust receiving means may conveniently be a sieve with a relatively coarse mesh size, greater than the size of the individual dust particles. Because of the tendency of dust to form loosely bound agglomerates the dust will be retained on the sieve to a considerable extent until air is blown on to it by the fan. The sieve helps to provide some turbulence in any air stream directed on to it.
The standardized dust dislodging force may for example be mechanical agitation provided by an object such as a duster coming into contact with the surface carrying the dust. The standardisation may be provided by specifying the nature of the object, the force applied, and the number of times the object is passed over the surface. Where any individual dust dislodging step is likely to be have be variable, as when a human tester uses a duster, a standardized dust dislodging force may be taken to be applied if the average of a sufficient number of individual results are combined to produce the final comparison.
For a more exact application of a standardized dust dislodging force it is preferred to use a stream of gas. The stream of gas, which is most conveniently air, must be such as to be capable of displacing dust from the surface on which the dust is deposited. The gas stream is conveniently produced by a compressor and is conveniently applied by a nozzle close to the surface from which dust is to be displaced. The nozzle may be disposed so as to apply a stream of gas parallel to the surface on which the dust is deposited. Alternatively it may be applied substantially perpendicularly to the surface. The gas stream must not be so weak that no significant amount of dust is displaced from any surface tested, or so strong that all the dust is displaced from all the surfaces tested. Once the inventive idea has been disclosed the skilled person will be able to select gas pressures, nozzle sizes and position relative to the surface, and duration so as to give the optimum discrimination between the surfaces to be tested. By way of example the nozzle may have a diameter of 3 to 10 mm, may be mounted 2-10 mm above the test panel and may be supplied with compressed air at a pressure of 0.5 to 2 bar (gauge).
The amount of dust in the atmosphere within the enclosure may be determined by any convenient method. Apparatus for determining the number of particles of various sizes dispersed in air is commercially available.
The excess dust particles released into the atmosphere of the enclosure by the action of the stream of gas may be removed by allowing them to settle out over a period of time. In such a case it will be necessary to provide a shield for any test surfaces within the enclosure which are awaiting examination in order to prevent this dust settling on them.